Recently, a pre-crash safety system has been developed in which position coordinate points and a relative velocity of another vehicle are obtained by a radar device and a risk of said another vehicle colliding with an own vehicle is calculated based on the movement history of the position coordinate points, such that appropriate safety measures are taken when it is determined that the risk is high.
The pre-crash safety system includes a radar device that obtains position coordinate points and a relative velocity of another vehicle, and an electronic control unit (ECU) that calculates, based on a movement history of the position coordinate points, a risk of said another vehicle colliding with an own vehicle and that causes a seat belt to be fastened and a brake to be applied when it is determined that the risk is high. In order to calculate the risk of said another vehicle colliding with the own vehicle, the ECU calculates a traveling direction vector, based on the movement history of the position coordinate points of said another vehicle.
A method for calculating the traveling direction vector is described with reference to FIG. 7.
FIG. 7 shows an example of the method for calculating the traveling direction vector.
As shown in (A) of FIG. 7, first, position coordinate points K obtained by the radar device are plotted in accordance with the order of acquisition thereof. Accordingly, a movement history of the position coordinate points is plotted. Next, as shown in (B) of FIG. 7, with regard to the movement history of the position coordinate points, linear function approximation is performed using, for example, the least square method. Thereby, a traveling direction vector 10 is generated.
As shown in (A) of FIG. 7, the position coordinate points K obtained by the radar device include normally recognized coordinate points K1, first extrapolation coordinate points K2, and second extrapolation coordinate points K3. The percentages of the normally recognized coordinate points K1, the first extrapolation coordinate points K2, and the second extrapolation coordinate points K3 and the arrangement thereof, which are shown in (A) of FIG. 7, are only an example and not limited thereto.
A normally recognized coordinate point K1 is a position coordinate point normally recognized by the radar device.
Calculation of the normally recognized coordinate point K1 requires the azimuth in which a target (hereinafter referred to as another vehicle) is located relative to the own vehicle, and the distance between said another vehicle and the own vehicle. The azimuth in which said another vehicle is located is, for example, represented by an angle θ between a straight line from the own vehicle to said another vehicle and a line representing the traveling direction of the own vehicle. Based on the measured values of the distance and the azimuth, the normally recognized coordinate point K1 can be calculated.
In a case where an FM-CW radar is used as the radar device, a distance R between the own vehicle and said another vehicle can be determined by using the following formula (1):R=C(ΔfU+ΔfD)/(8fmΔF)  formula (1),where the characters denote the following meanings:C: the velocity of light, ΔfU: the beat frequency in the up section of a modulation wave (for example, triangular wave), ΔfD: the beat frequency in the down section of the modulation wave, fm: the repetition frequency of the modulation wave, and ΔF: the amplitude of the modulation wave.
The angle θ can be measured by using, for example, a monopulse system. In this case, the angle θ can be calculated by using the following formula (2):θ=sin−1(λφ/(2πd))  formula (2),where the characters denote the following meanings:λ: the wavelength of a transmission wave, d: the distance between two antennas, and φ: the phase difference of a reflected wave received by the two antennas.
In a case where an FM-CW radar is used as the radar device, a relative velocity V of said another vehicle can be determined by using the following formula (3):V=±(ΔfU−ΔfD)/2  formula (3),
where the characters denote the following meanings:
ΔfU: the beat frequency in the up section of the modulation wave (for example, triangular wave), and ΔfD: the beat frequency in the down section of the modulation wave.
A first extrapolation coordinate point K2 is a position coordinate point estimated through first extrapolation processing. In the first extrapolation processing, in a case where the radar device performing periodical target detections has succeeded in detecting a position coordinate point and a relative velocity of said another vehicle in a previous detection cycle but has failed in detecting any of measurement parameters for specifying a position coordinate point and a relative velocity of said another vehicle in a current detection cycle, the radar device estimates the position coordinate point and the relative velocity of the current detection cycle, based on values of the measurement parameters for said another vehicle which are obtained in the previous detection cycle.
The first extrapolation processing is performed in a case where, in the current detection cycle, the radar device has measured, as the measurement parameters, neither the beat frequency ΔfU of the up section nor the beat frequency ΔfD of the down section. The beat frequency ΔfU of the up section and the beat frequency ΔfD of the down section which are obtained in the previous detection cycle may be actually measured values or estimated values. In a case where the beat frequency ΔfU of the up section and the beat frequency ΔfD of the down section which are obtained in the previous detection cycle are estimated values, first extrapolation coordinate points K2 may be obtained in succession, or a first extrapolation coordinate point K2 and a second extrapolation coordinate point K3 may be obtained in succession.
A second extrapolation coordinate point is a position coordinate point estimated through second extrapolation processing. In the second extrapolation processing, in a case where the radar device performing periodical target detections has succeeded in detecting a position coordinate point and a relative velocity of said another vehicle in a previous detection cycle but has failed in detecting some of the measurement parameters for specifying a position coordinate point and a relative velocity of said another vehicle in a current detection cycle, the radar device estimates the position coordinate point and a relative velocity of the current detection cycle, based on the values of the measurement parameters for said another vehicle which are obtained in the previous detection cycle.
The second extrapolation processing is performed in a case where, in the current detection cycle, the radar device has failed in measuring, as the measurement parameters, either one of the beat frequency ΔfU of the up section and the beat frequency ΔfD of the down section. Estimation of a position coordinate point and a relative velocity through the second extrapolation processing requires, in order to make up the beat frequency that has not been measured, a beat frequency obtained in a previous detection cycle. The beat frequency obtained in the previous detection cycle may be an actually measured beat frequency or an estimated beat frequency. When the beat frequency obtained in the previous detection cycle is an estimated beat frequency, second extrapolation coordinate points K3 may be obtained in succession, or a first extrapolation coordinate point K2 and a second extrapolation coordinate point K3 may be obtained in succession.
FIG. 8 is a diagram illustrating a relationship between: the normally recognized coordinate point, the first extrapolation coordinate point and the second extrapolation coordinate point; and the azimuth in which another vehicle is located, the relative velocity of said another vehicle and the distance between said another vehicle and the own vehicle. A circle denotes that the corresponding measurement parameters have been normally measured by the radar device. A triangle denotes that some of the parameters necessary for the radar device to measure the relative velocity and the distance have not been measured. A cross denotes that none of the parameters necessary for the radar device to measure the relative velocity and the distance have been measured.
As shown in FIG. 8, a first extrapolation coordinate point K2 is calculated in a case where the azimuth θ has not been measured and none of the parameters (the beat frequency ΔfU of the up section and the beat frequency ΔfD of the down section) necessary to measure the distance R and the relative velocity V have been measured. A second extrapolation coordinate point K3 is calculated in a case where the azimuth θ has been measured but some of the parameters necessary to measure the distance R and the relative velocity V (either one of the beat frequency ΔfU of the up section and the beat frequency ΔfD of the down section) have not been measured.
As described above, the position coordinate points K obtained by the radar device include normally recognized coordinate points K1, first extrapolation coordinate points K2, and second extrapolation coordinate points K3. Since the normally recognized coordinate points K1 are highly reliable, in a case where a group of the position coordinate points consists only of the normally recognized coordinate points K1, the reliability of the traveling direction vector 10 is also high. On the other hand, the first extrapolation coordinate points K2 and the second extrapolation coordinate points K3, which are estimated coordinate points, are less reliable. Therefore, the reliability of the traveling direction vector 10 is lowered in accordance with an increase of the percentages of the first extrapolation coordinate points K2 and the second extrapolation coordinate points K3 in the group of the position coordinate points. A collision prediction made based on a less reliable traveling direction vector 10 may more likely to lead to a wrong prediction. On the other hand, generation of a traveling direction vector 10 without using extrapolation coordinate points may result in a delayed generation of the traveling direction vector 10 and thus a delayed collision prediction, whereby measures against a collision may not be taken in advance.
Patent Document 1 discloses a system in which position coordinate points of another vehicle are obtained by a radar device and a traveling direction vector is calculated based on the movement history of the position coordinate points, so as to make a collision prediction about the collision between said another vehicle and the own vehicle. However, since the reliability of the traveling direction vector is not calculated, a prediction that there will be a collision may be made even when the possibility of the collision is actually low, which may result in actuation of a device that takes safety measures.    Patent Document 1: Japanese Laid-open Patent Publication No. 2007-279892